1. Field of the Invention
This invention relates to systems for collecting and condensing light from a non-circular source to a circular target for illumination and projection systems.
2. Background of the Invention
An objective of systems that collect, condense, and couple electromagnetic radiation into a standard waveguide, such as a single fiber or fiber bundle, or that output electromagnetic radiation to a homogenizer of a projector, is to maximize the brightness of the electromagnetic radiation at a target. There are several common systems for collecting and condensing light from a lamp for such illumination and projection systems. Some of these may generally be classified as on-axis systems, in that the lamp, the target, and the optical axis of the reflector are co-linear. Others may be classified as off-axis systems, in that some of the components are not on the optical axis 6.
Ellipsoid reflectors and parabolic reflectors may be used together with imaging lenses in on-axis projection systems. Among the useful characteristics of these systems is their rotational symmetry. Since an output target, e.g. an optical fiber bundle, may also be round, the design is conceptually simplistic and intuitively attractive. These systems, however, suffer from lost brightness at the coupling. This loss of brightness degrades the overall efficiency of the projection system.
FIG. 1 shows a related lamp/reflector configuration in which a lamp 2 is placed at a focus of a parabolic reflector 4. The parabolic reflector 4 collimates reflected light such that the light collected by the reflector 4 is parallel to an optical axis 6. A focusing lens 8 is used to collect the collimated beam and focus the light into a target 10. The configuration shown in FIG. 1 may be seen to have rotational symmetry about the optical axis.
FIG. 2 shows another related lamp/reflector configuration in which a lamp 12 is placed at a first focus 14 of an ellipsoid reflector 16 and a target 18 is placed at a second focus 20. This configuration possesses rotational symmetry about an optical axis 22 as well.
The configurations shown in FIGS. 1 and 2 are on-axis systems, since the components are, in general, aligned along an optical axis. A typical intensity profile 30 of the output of such systems is shown in FIG. 3. The output may be seen to possess rotational symmetry as well, with an intensity peak 32 at the center. Due to the nature of these two systems, however, light emitted at various angles from the lamp is magnified differently. The brightness of the light is thus diminished at the target.
A collecting and condensing system such as that shown in FIG. 4, known as an off-axis system, may be used to produce 1:1 magnification of the light at a target. The system shown in FIG. 4 consists of a lamp 42, a primary reflector 44, and a target 46. A retro-reflector 48 may be used to increase the overall efficiency and brightness of the system.
A cross-section of a collecting and condensing system using two symmetric parabolic reflectors is shown in FIG. 5. Light emitted from a lamp 52 is collected and collimated by the first parabolic reflector 54. Rays a, b, and c illustrate three possible paths the light may take from the lamp to a target. Ray a, which has the shortest distance to travel to the first parabolic reflector 54, has the highest divergence angle of the three. Ray c, on the other hand, has the shortest distance to travel to the first parabolic reflector 54 but produces the smallest divergence angle. Ray b is shown to be in the middle and has a divergence angle in the middle of the range.
Rays a, b, and c are reflected at locations on the second parabolic reflector 56 that corresponding to their reflections on the first parabolic reflector 54. The distance traveled by each ray from the second parabolic reflector 56 to the target 58 is thus the same as the corresponding distance between the lamp 52 and the first parabolic reflector 54. Each ray may consequently be focused onto the target 58 with similar divergence at each reflector. The brightness of the arc at the target is preserved as a result of unit magnification. Neither of the configurations shown in FIG. 4 nor 5, however, possess rotational symmetry about an optical axis.
Neither of the configurations shown in FIG. 4 nor 5 possess rotational symmetry. The image of the arc at the target is the image of the arc viewed from the side, and thus bears the same length and width as the arc itself.
FIG. 6 shows the non-symmetrical intensity profile 60 of the image of an arc at the target of either of the configurations shown in FIG. 4 or 5. It would be desirable for a shape of a target to match the non-symmetric intensity profile of the image of the arc.
U.S. Pat. No. 4,757,431, e.g, the disclosure of which is incorporated by reference, describes an improved condensing and collecting system employing an off-axis spherical concave reflector. Such a system enhances the maximum flux that illuminates a small target and thus the amount of flux density collectable by the target. U.S. Pat. No. 5,414,600, the disclosure of which is incorporated by reference, in which the concave reflector is an ellipsoid, and U.S. Pat. No. 5,430,634, the disclosure of which is incorporated by reference, in which the concave reflector is a toroid, improved further on this system.
These systems provide 1:1 magnification of the light source at the target, thus preserving the brightness of the arc. The image of the arc, however, is presented at the target. Since the image of the arc is not usually circular, it does not necessarily match well with the target. Arcs are generally approximately elliptical in shape, and possess a certain aspect ratio. This aspect ratio is generally proportional to the length of the arc, so that longer arcs have larger aspect ratios. As a result, the image of the arc at the target may not be optimized for coupling into, e.g. a round optical fiber or a projection engine.
It may also be desirable to match the light incident on a target to the numerical aperture (NA) of a target. The NA of a target, e.g. an output fiber, is related to the angle of the acceptance cone of the light being received. The NA may thus determine how much of the incident light is actually coupled into the output fiber. In the case of a projection engine, e.g. the projection lens and related optical train may determine the NA at the light entrance. It may also be desirable for maximum collection efficiency for the light from the lamp to have an NA similar to that of the target.
In FIG. 7 is shown a geometrical representation of the angles of emission of light from an arc lamp. The axis of the arc is assumed to be on the y-axis. The two emission angles are Θx and Θy. The angle of emission Θx of an arc generally extends about 45 degrees above and below the x-z plane, while Θy encompasses a full 360-degree circle around the y-axis. The light from the arc may be seen to be non-symmetrical when viewed from a point of view in the x-z plane.
The light from the arc may further be seen to have an aspect ratio greater than one when viewed from the side, i.e. from a point of view in the x-z plane. A reflector can be designed to capture all this light and focus it into a target. It may be desirable, however, for the reflector to cooperate with beam transforming optics such that when the collected light is coupled into the input aperture of the target it is actually useable.
FIG. 8 shows various configurations of input apertures for a target. The input apertures generally have aspect ratios greater than one. The aspect ratios of the input apertures may thus be made to be similar to the aspect ratio of the emission area of an arc lamp viewed from the side. Matching an input aperture at the target to an arc, however, does not necessarily match it with the final output device, e.g. a fiber or projection engine. It would be desirable, therefore, for a transforming device to transform the aspect ratio and the NA of the input light into a satisfactory aspect ratio and NA for the output device.
FIG. 9 shows the output of a typical arc lamp. The light output may be seen to be within a 90° angle in the direction along the axis of the lamp and 360° around the lamp. In using the dual paraboloid or dual ellipsoid reflector configurations with retro-reflectors, the focused light at the target may have a numerical aperture (NA) of 1.0 in the z-direction and 0.7 in the x-direction and y-direction as shown in FIG. 10. These coupling systems do not have a rotational symmetry and the resulting NA may be rectangular.
In practice, light with such a large NA has to be transformed such that the NA is smaller and the area is larger following the brightness principle. FIG. 11 shows a typical tapered light pipe that does such a transformation. Following the brightness principle, the relationship is:d1×NA1=d2×NA2
Normally, the light pipe is designed such that the output NAs are the same in both directions as shown in FIG. 12.
FIG. 13 shows a three-quarter view of the light pipe shown in FIG. 11. Due to the finite length of the light pipe, the light exiting the light pipe does not follow the formula exactly and the output NA is usually larger than theory would predict. Further, the output is generally not telecentric.